1. Field of the Invention
This invention is directed to an apparatus and method using low-voltage and/or low-current scanning probe lithography and, more particularly, to an apparatus and method for performing low-voltage and/or low-current scanning probe lithography using a sharp tip or probe attached to a cantilever to maintain contact with an imaging layer to be lithographed.
2. Description of the Related Art
Microelectronic and micromechanical fabrication technology continues to move toward smaller feature sizes (and hence, higher lithography resolution) for integrated circuits, lithographic masks or micromechanical components. At present, focussed beam lithography is used to achieve the highest lithographic resolution by direct patterning on a substrate, or by patterning a mask to be used for lithography in conjunction with optical, ultraviolet or x-ray radiation. However, as discussed below, there are limits to focussed beam lithographic resolution as currently practiced.
The common approach to achieving smaller lithographic resolution uses focussed beam lithography with high electron or ion beam energies, to obtain a more tightly focussed spot in which the beam is projected onto a mask surface or substrate. To form a pattern on a mask or chip using such conventional focussed beam lithographic devices or techniques, a suitable substrate or mask is first covered with a thin layer of radiation sensitive material (i.e., a resist). The electron or ion beam exposes (i.e., induces a physical change in) the resist and the exposed resist pattern can be developed using a suitable solvent.
For feature sizes less than 100 nanometers, lithographic resolution is limited not by the focus of the beam but by electron or ion beam scattering in the resist and from the substrate. This scattering causes reduced lithographic resolution and pattern distortions commonly referred to as "proximity effects". Although progress has been made over the last twenty years in reducing these effects, such effects are an inherent manifestation of the interaction of the high-energy electrons or ions used in a beam with solid matter, such as a substrate or mask surface. Thus, despite the use of beams with energies as high as 100 kilo-electron volts (keV), scattering (although more diffuse) still occurs and limits the lithographic resolution of the resist. These proximity effects are particularly acute with resists which have sensitivities which could be expected to be used to obtain practical throughput levels for integrated circuits, lithographic masks or micromechanical devices, for example.
A different approach to achieve higher lithographic resolution is to reduce the energy of the electron or ion beam used for beam lithography. One conventional device employing this technique uses a scanning tunneling microscope (STM) which uses field emission of electrons from an electrode tip. From the electrode tip, the electrons travel through a resist to a substrate, to expose the resist. This conventional STM suffers from the disadvantage that the current flowing from the tip of the electrode to the substrate via the resist, is used both to expose the resist and to control the separation between the electrode and the substrate. By coupling these functions, the conventional STM is operationally limited. Specifically, the operation of the conventional STM with less than 15 volts between the electrode and the substrate is difficult because the control servo system pushes the electrode too close to the sample and into the resist. As a result, the tip can penetrate the resist and only the resist volume beneath the electrode is exposed and utilized. Also, this feature of the conventional STM places conditions upon the environment in which lithography can be performed. At 15 volts between the electrode and the substrate, vacuum conditions are required to prevent interaction between ambient gases and the substrate due to the electrons emitted between the electrode and the substrate. Because the typical energy needed to affect a modification in a resist is only on the order of 1 electron-volt, it would be desirable to provide operation at less than 15 volts to provide lithographic capability in ambient conditions or under an inert atmosphere such as argon or an inert liquid such as an oil or paraffin. Such operation would eliminate the need for expensive equipment to provide high-vacuum conditions as required with conventional STM devices as well as with high-energy beam lithography.
Another technology related to the present invention is scanning (or atomic) force microscopy ("SFM" or "AFM"). Scanning force micropsy has been used to map the surface topography of a sample by dragging an insulator tip over the sample. AFMs have also been used to image the surface of a sample for registration and alignment purposes. The insulator tip position is controlled by maintaining a constant force of the tip on the sample, and the deflection of a cantilever to which the insulator tip is attached is monitored to map the surface topography of the sample.